/*************************************************************************/
/*                                                                       */
/*  Copyright (c) 1994 Stanford University                               */
/*                                                                       */
/*  All rights reserved.                                                 */
/*                                                                       */
/*  Permission is given to use, copy, and modify this software for any   */
/*  non-commercial purpose as long as this copyright notice is not       */
/*  removed.  All other uses, including redistribution in whole or in    */
/*  part, are forbidden without prior written permission.                */
/*                                                                       */
/*  This software is provided with absolutely no warranty and no         */
/*  support.                                                             */
/*                                                                       */
/*************************************************************************/

EXTERN_ENV

#include <math.h> 

/* return the value of a with the same sign as b */

#define sign(a,b)  (b < 0 ) ? ( (a < 0) ? a : -a) : ( (a < 0) ? -a : a) 

CSHIFT(XA,XB,XMA,XMB,XL,BOXH,BOXL)
  /* compute some relevant distances between the two input molecules to
     this routine. if they are greater than the cutoff radius, compute
     these distances as if one of the particles were at its mirror image
     (periodic boundary conditions).
     used by the intermolecular interactions routines */
  
  double XA[], XB[], XL[];
  double BOXH, BOXL, XMA, XMB;
{
    
    int I;
    
    XL[0] = XMA-XMB;
    XL[1] = XMA-XB[0];
    XL[2] = XMA-XB[2];
    XL[3] = XA[0]-XMB;
    XL[4] = XA[2]-XMB;
    XL[5] = XA[0]-XB[0];
    XL[6] = XA[0]-XB[2];
    XL[7] = XA[2]-XB[0];
    XL[8] = XA[2]-XB[2];
    XL[9] = XA[1]-XB[1];
    XL[10] = XA[1]-XB[0];
    XL[11] = XA[1]-XB[2];
    XL[12] = XA[0]-XB[1];
    XL[13] = XA[2]-XB[1];
    
    /* go through all 14 distances computed */
    for (I = 0; I <  14; I++) {
        /* if the value is greater than the cutoff radius */
        if (fabs(XL[I]) > BOXH) {
            XL[I]  =  XL[I] -(sign(BOXL,XL[I]));
        }
    } /* for */
} /* end of subroutine CSHIFT */

